1. Field of the Invention
The present invention relates to an integral dielectric filter and dielectric duplexer using a dielectric block and communications equipment provided with the filter and duplexer, which are mounted on mobile communications devices, etc.
2. Description of the Related Art
The construction of a prior art dielectric filter is described with reference to FIGS. 8A to 8C.
A dielectric block 1 in the form of a substantially rectangular solid is provided with the through holes 2a to 2c on which respective internal conductors 3a to 3c are formed to define respective resonant cavities. Excitation holes 9 and 10 also extend through the dielectic block 1 and have respective internal electrodes formed thereon. An external conductor 4 is formed on the substantially entire outside surface of the dielectric block 1. Internal-conductor-free portions 33a to 33c are provided in the through holes 2a to 2c to define open ends of the resonant cavities. The side surface shown in FIG. 8c is referred to hereinafter as the open end surface of the dielectric block (because it is adjacent the conductor-free portion 33a-33c). The internal conductors of the other end of the resonant cavities are coupled to the external electrode 4 to form short-circuited sides of the resonant cavities. The side surface of the dielectric block 1 shown in FIG. 8a will be referred to hereinafter as the closed side surface. Each of the through holes 2a to 2c is constructed as a stepped hole in which the inner diameter on the open circuited side of the cavities is different from the inner diameter on the side of the short-circuited side thereof.
A pair of external-conductor-free portions 7 and 8 extend from the closed side surface to the bottom or mounting surface of the dielectric block 1 to define input-output electrodes 5 and 6 which are conductivity coupled to one end of the internal electrodes of the excitation holes 9 and 10. The other end of the inside electrode of the excitation holes 9 and 10 is conductivity coupled to the external conductor 4.
An integral dielectric filter using a dielectric block is constructed such that the input-output electrodes 5 and 6 provided with the excitation holes 9 and 10 are electromagnetically coupled to the resonator cavities defined by the internal conductors 3a to 3c. 
However, this filter exhibits the following problems.
FIG. 9 shows the grounding current on the open end surface of the dielectric block. When a signal is input to the dielectric filter, the TEM mode as a basic mode is resonated and propagated. Because of this signal, a ground current flows from each of the internal conductors and the inside electrodes formed in the excitation holes to the external conductor which acts, as a grounding electrode.
The grounding current flowing from the inside electrode of the excitation hole 9 to the external conductor flows from the opening end of the excitation hole 9 toward the mounting (lower) surface and its opposite (upper) surface as shown in FIG. 9.
On the other hand, since the external-conductor-free portions 7 and 8 surrounding the input-output electrodes 5 and 6 extend over the mounting surface, the electromagnetic field distribution becomes asymmetrical on the mounting surface and its opposite surface of the dielectric filter. As a result, TE modes having TE101 mode, having a component of electric field perpendicular to the mounting surface and its opposite surface and revolving when seen from the mounting surface, as a main component are excited. As a result, the attenuation and spurious characteristics around the passband of the dielectric filter are greatly worsened.
It is an object of the present invention to construct a dielectric filter and dielectric duplexer of a simple construction having excellent spurious characteristics by reducing unnecessary modes except TEM mode as a basic mode and communications equipment provided with the dielectric filter and duplexer.
To this end, the invention comprises:
a dielectric block having opposing open and closed end faces extending between opposing top and bottom surfaces thereof;
an outer conductor formed on an outer surface of the dielectric block including the top and bottom surfaces and the closed end face thereof;
a plurality of plated through holes extending from the front to the rear face of the dielectric block, the plated through holes having respective internal conductors formed thereon, the internal conductors being conductively coupled to the outer conductor at the closed end face of the dielectric block;
an excitation hole holes extending from the open to the closed end face of the dielectric block and having an inside electrode formed thereon, the inside electrode being conductively coupled to the outer conductor at the open end face thereof;
an input-output electrode extending over the open end face of the dielectric block and the mounting surface of the dielectric block, the input-output electrode being conductively coupled to the inside electrode; and
ground current setting means for controlling the ground current flowing through the outer conductor located on the open end surface of the dielectric block from the inside electrode of the excitation hole to the top and mounting surfaces of the dielectric block.
In accordance with one preferred embodiment of the invention, the outer conductor is located on the open end face of the dielectric block and the ground current setting means is an electrode-free portion provided on the open end face at a location between one end of the excitation hole and the top and/or mounting surfaces.
In accordance with another preferred embodiment of the invention, the ground current setting means is the difference in the distance of the excitation hole from the mounting surface at the open end face relative to the distance of the excitation hole from the mounting surfaces at the closed end face, the distances being measured in a direction perpendicular to the mounting surface.
In accordance with another preferred embodiment of the invention, the ground current setting means is a recess formed in the open end face of the dielectric block in the area surrounding the end of the excitation hole which opens at the open end face. In one embodiment, the recess extends at an oblique angle to the mounting surface so as to be deeper at the top surface of the dielectric block than at the mounting surface thereof.
The invention is further directed towards a dielectric duplexer containing the foregoing dielectric filters.
The invention is further directed towards communications equipment containing the foregoing dielectric filters and duplexers.